Sum: S = 1 + 2x + 3x ^ 2 +... + NX ^ (n-1)

If x = 1
Then s = 1 + 2 + +n=n(n+1)/2
If x is not equal to 1
Then XS = x + 2x ^ 2 + 3x ^ 3 + +(n-1)x^(n-1)+nx^n
s-xs=1+x+x^2+x^3+…… +x^(n-1)-nx^n
=1*(1-x^n)/(1-x)-nx^n
So s = (1-x ^ n) / (1-x) ^ 2-nx ^ n / (1-x)

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